Mating triggers the development of new neurons in the smell centre of
the brain, a finding researchers hope will provide clues on how to trick
other parts of the vital organ to repair themselves after injuries caused
by strokes or head traumas. The possibility hinges on the discovery that
a naturally occurring hormone called prolactin - production of which surges
after sex and during pregnancy - prompts stem cells in the brain to produce
new neurons in the brain's olfactory bulb.

"The importance of it, beyond the basic biology of stem cells, is the
fact that prolactin may be an important neurogenic molecule that may have
significant potential for generating new brain cells," said Samuel Weiss,
an expert in neurological stem cell biology in the University of Calgary's
faculty of medicine.

Weiss is the senior author of the study, published Friday in the journal
Science. The work was a collaboration between his laboratory and that of
James Cross, also part of the genes and development research group at the
University of Calgary.

The researchers were building on a discovery Weiss reported about a
decade ago, that the brains of adult mice contain stem cells which are
the body's building blocks. Stem cells have the capacity to turn into any
type of cell. Later it was shown that the adult human brain contains stems
cells as well.

Those findings were considered monumental, because it had long been
believed that the brain cannot generate new cells to replace or repair
those lost to injury.

Over time, Weiss and his colleagues determined that the major role of
these stem cells was to produce new neurons - the brain cells that communicate
with one another - for the olfactory bulb.

The stem cells are located in another part of the brain and the neurons
they produce have to migrate to the olfactory bulb. Once there they "differentiate,"
meaning they mature into a specific type of cell - in this case olfactory
bulb neurons.

Further work showed that female mice which were genetically engineered
to contain fewer stem cells - and therefore fewer new neurons in the olfactory
bulb - were less attentive mothers to their offspring.

It is well established that the sense of smell is crucial to mating
and to rearing offspring, allowing both animals and insects to recognize
their mates and progeny.

Working with mice, the Calgary researchers determined that there was
a spike in olfactory bulb neuron production during the early stages of
pregnancy and shortly after birth. The hormone which appears to trigger
the production is prolactin, which fuels several key physiological changes
in females during pregnancy and after birth.

The hormone is also present in males, where it obviously serves a different
function.

Weiss and his colleagues discovered that prolactin levels - and consequently
new neuron production - surge after mating as well. And they found that
if prolactin was introduced to the blood system by injection, it also stimulated
production of new neurons for the olfactory bulb.

While they aren't certain the same process occurs in human, they do
know that sex triggers a surge of prolactin in humans.

Weiss acknowledged that some researchers will be interested in their
findings because of what they say about mating and the sense of smell.

But he and his colleagues want to see whether the process they've found
can be altered to send new neurons to other parts of the brain. Another
study, published last year in the journal Nature, suggests it may be possible.

Researchers working with rats reported that in the case of brain damage
some of the stem cells spontaneously redirected new neurons towards the
injury, but in numbers too small to repair the damage.

If the stem cells could be tricked into directing sufficient numbers
of new neurons, "it provides potentially a way to boost the brain's self-repair
process and allow for enhanced functional recovery after the stroke," Weiss
said, who has been collaborating with Bryan Kolb at the University of Lethbridge
to test the theory.

"We have exciting preliminary data suggesting that the new stem cell-generated
brain cells can be redirected to parts of the rodent brain that are damaged
after a stroke and this results in partial improvement of the animal's
ability to move its limbs," he said.

If the theory is proven, it might also provide clues on how to mitigate
the damage done by neurodegenerative diseases such as Parkinson's or Alzheimers.

Funding for the work was provided by the Canadian Institutes of Health
Research, the Canadian Stroke Network, the Alberta Heritage Foundation
for Medical Research, the Multiple Sclerosis Society of Canada and the
Stem Cell Network of the Network of Centres of Excellence.